1. Field of the Invention
The present invention relates to stabilization of polyvinyl chloride resins and polyvinyl chloride resin compositions and further relates to stabilized resins.
2. Description of the Prior Art
Halogen-containing polymers, sych as polyvinyl chloride homopolymers and copolymers, are useful because of their desirable physical properties. During their fabrication into plastic sheets, rigid bodies, and the like, vinyl resins, however, are susceptible to discoloration due to heat degradation. This is especially troublesome in the course of compounding these resins at elevated temperatures. At these temperatures, which can be as high as 180 to 200.degree. C, such resins are susceptible to heat degradation which is manifested in discoloration and loss in mechanical properties.
Many heat stabilizers are known to the art. Among the more important heat stabilizers in commerical use are metal chelates of dicarbonyl compounds or mixtures of metal chelates of dicarbonyl compounds with salts of carboxylic acids of calcium, lead and cadmium such as disclosed in U.S. Pat. Nos. 2,307,075 and 2,669,548. Salts of heavy metals and higher fatty acids have also been used in combinations with organic phoshites (U.S. Pat. No. 2,564,646) or with polyols (U.S. Pat. No. 2,711,411). Although these compounds have been successful in providing good stability, many of these compounds impart or do not entirely prevent an early yellow discoloration to the resin, which is manifested before severe heat deterioration occurs. This early discoloration has not been considered disadvantageous for many uses and the efforts of most research in this field has been directed towards minimizing the onset of the more serious heat deteriorations, i.e., blackening which occurs during long heating as in certain continuous milling operations. However, because of this early discoloration and accompanying haziness or cloudiness that may also appear, it has not been possible in all cases to obtain a substantially clear and colorless polyvinyl chloride resin composition. Therefore, a need exists in the polyvinyl chloride resin art for a stabilizer composition when added to polyvinyl chloride homopolymer and copolymer resin compositions which will impart to the resins the desired combination of heat and light stability, clarity, transparency, compatability, resistance to exudation and other valuable properties.